Mobile telephony systems, in which user equipment such as mobile handsets communicate via wireless links to a network of base stations connected to a telecommunications network, have undergone rapid development through a number of generations. The initial deployment of systems using analogue modulation has been be superseded by second generation digital systems, which are themselves currently being superseded by third generation digital systems such as UMTS and CDMA. Third generation standards provide for a greater throughput of data than is provided by second generation systems; this trend is continued with the proposal by the Third Generation Partnership Project of the so-called Long Term Evolution system, often simply called LTE, which offers potentially greater capacity still, by the use of wider frequency bands, spectrally efficient modulation techniques and potentially also the exploitation of spatially diverse propagation paths to increase capacity (Multiple In Multiple Out).
Distinct from mobile telephony systems, wireless access systems have also undergone development, initially aimed at providing the “last mile” (or thereabouts) connection between user equipment at a subscriber's premises and the public switched telephone network (PSTN). Such user equipment is typically a terminal to which a telephone or computer is connected, and with early systems there was no provision for mobility or roaming of the user equipment between base stations. However, the WiMax standard (IEEE 802.16) has provided a means for such terminals to connect to the PSTN via high data rate wireless access systems.
Whilst WiMax and LTE have evolved via different routes, both can be characterised as high capacity wireless data systems that serve a similar purpose, typically using similar technology, and in addition both are deployed in a cellular layout as cellular wireless systems. Typically such cellular wireless systems comprise user equipment such as mobile telephony handsets or wireless terminals, a number of base stations, each potentially communicating over what are termed access links with many user equipments located in a coverage area known as a cell, and a two way connection, known as backhaul, between each base station and a telecommunications network such as the PSTN.
As the data capacity of cellular wireless systems increases, this in turn places increasing demands on the capacity of the backhaul, since this is the connection that has to convey the wireless-originating traffic to its destination, often in an entirely different network. For earlier generations of cellular wireless systems, the backhaul has been provided by one or more connections leased from another telecommunications operator (where such a connection exists near to the base station); however, in view of the increasing data rates, the number of leased lines that is required is also increasing. Consequently, the operational expense associated with adopting multiple leased lines has also increased, making this a potentially expensive option for high capacity systems.
As an alternative to leased lines, dedicated backhaul links can be provided by a variety of methods including microwave links or optical fibre links. However each of these methods of backhaul has associated costs. Dedicated fibre links can be expensive in terms of capital expense due mainly to the cost of the civil works in installation, and this problem is especially acute in urban areas. Microwave links also involve the capital expense of equipment and require expert installation due to narrow beam widths leading to the requirement for precise alignment of antennas.
As an alternative to the provision of a dedicated backhaul link for each individual base station, it is possible to use the radio resource of the cellular wireless system to relay backhaul traffic from one base station to another. Typically, the base station using the cellular radio resource for backhaul is a small low power base station with an omnidirectional antenna known as a relay node. Such a system can be used to extend the area of cellular wireless coverage beyond the area of coverage of conventional base stations that are already equipped with a dedicated backhaul.
FIG. 1 shows a conventional relay node operating within a cellular wireless network; the operation may for example be in accordance with IEEE 802.16j. A user equipment 5b is in communication with a relay node base station 3. As already mentioned, the relay node typically employs an omnidirectional antenna giving a uniform radiation pattern 15 in azimuth. As the relay node 3 is not provided with a backhaul link separate from the cellular wireless resource, the relay node is allocated radio resource timeslots for use relaying backhaul data to and from the adjacent base station 1 which is itself connected by microwave link to a microwave station 7 and thence to a telecommunications network such as the public switched telephone network The base station 1 in this example employs a conventional tri-cellular coverage scheme; three antennas are each connected to a radio transceiver at the base station and the radiation pattern consists of three lobes 11a, 11b and 11c. A user equipment 5a is shown in communication with the base station 1 via antenna pattern lobe 11c. It should be noted that the antennas employed by the base station 1 to give the tri-sectored coverage scheme are optimized to give coverage within this particular cellular scheme; as a result there are regions between the antenna lobes 11a, 11b, 11c with little coverage from the base station, as these areas are designed to be covered by a neighbouring base station. So it can be seen that the base station antennas are not designed to give even coverage over 360 degrees from an individual base station. Also, the antennas on the tri-cellular base station 1 are given a deliberate down-tilt of several degrees to give good coverage within the sector in question while minimizing interference with other base stations. As a result, this antenna arrangement may not be ideal for communication with a relay node 3 outside the normal area of coverage of the base station 1, as, depending on its location, the relay node may fall in a null between the lobes in the tri-cellular radiation pattern and in addition the down-tilt of the antennas may reduce gain beyond a certain distance from the base station.
FIG. 2 shows a time frame structure allocating timeslots alternately to access 17a . . . 17d and backhaul 19a . . . 19c. Typically, all of the access payload data will be relayed by the backhaul link; if the spectral efficiency of the backhaul and access links is the same, then the access and backhaul timeslots will occupy approximately equal amounts of time. There may be a significant reduction in capacity available in the access links to the user equipment due to the need to reserve timeslots for backhaul. This problem is exacerbated in the IEEE 802.16j scheme, where the allocation of timeslots to backhaul is localized around the area of the relay node.
Hence it can be seen that backhaul links for high capacity cellular wireless systems can present a significant expense; to mitigate this, the cellular wireless resource can be used to relay backhaul links from one base station to another, but when employed in conventional arrangements, this incurs significant limitations to data capacity and restrictions on the positioning of base stations.